1. Field of the Invention
The invention relates to a signal processing method including steps for acquiring signals relating to an object having moving parts, and steps for constructing a sequence of images on the basis of said signals. The invention also relates to an echographic device for carrying out this method.
2. Description of the Related Art
The invention is used in the field of medical echographic imaging, for providing cardio-vascular diagnostic tools for non-invasive study of anomalies of arteries and notably of stenoses. A diagnostic criterion for a stenosis is an abrupt reduction of an artery diameter. Another criterion is the blood velocity in the stenosed zone. Studies have shown that the behavior of blood in a stenosed artery does not satisfy Bernouilli's law concerning fluid flows in a sense that flow velocity does not increase in conformity with said law in the zone in which the artery diameter decreases. To the contrary, the blood flow velocity decreases as soon as the artery diameter reaches a stricture threshold. Consequently, the artery gradually becomes completely occluded in the zone which is first only constricted, ultimately causing a thrombosis phenomenon. Rigid plaques can also appear in the layers of the arterial walls causing changes of the wall elasticity and ultimately causing so large wall tensions that during the cardiac cycle the arterial wall is distorted to the point of rupture and may emit emboli.
Therefore, the medical field has a need for non-invasive means for studying arteries, notably stenosed arteries, in order to find an explanation for this behavior.
A method for determining blood flow velocity by Color Velocity Imaging noted CVI is already known from a publication entitled "CVI challenges Doppler in Vascular pathology" by M. CLAUDON, D. M., in "Diagnostic Imaging International, Vol. 7, pp.61-65, 1991, Miller Freeman Publication". This paper discloses that Color Velocity Imaging is a new non-Doppler ultrasound technique for vascular flow color imaging. The first imaging technique called Doppler calculates flow velocity indirectly by measuring the frequency shift and the phase shift variations from the original transmitted frequency pulse of the Doppler transducer. These shifts are caused by movement of blood cell clusters. A physical limitations of this principle is that the transmitted central probe frequency also shift due to uncontrollable phenomena linked with tissue structure which disturbs the accuracy of the blood velocity computation. Color Doppler systems minimise this error by averaging a high number of measurements, which results in decreasing the number of color lines being processed, in order to obtain an acceptable frame rate. Spatial and true hemodynamic resolutions are reduced and therefore the degree of clinical information available is limited. In addition the use of longer pulse lengths reduces the axial resolution of color Doppler images as compared to gray-scale images.
Instead of measuring the frequency shift as in Doppler imaging technique, the second imaging technique called CVI, measures flow velocity directly by using time domain processing. CVI tracks individual clusters of blood cells using ultrasound to measure the distance and the time travelled. CVI is based on the principle of signature recognition. In a time domain correlation process, a first echo is stored in digital memory. The shape of signal traces the relative position of blood cells which is called ultrasound signature of the cluster. Some microseconds later, a second echo signal is stored. A computing system analyses the two signal signatures by time shifting these two signatures until they match perfectly. This time shift is directly related to the distance the blood cells have moved, using the speed of sound in tissue. Blood-cell velocity is obtained by dividing this measured distance by the time between the two corresponding ultrasound pulses. Correction of the cosine angle between the vessel axis and the ultrasound beam is the same as with Doppler systems. The velocity computations are coded in shades of red and blue according to the direction of blood flow. Unlike Color Doppler, CIV measures peak velocities instead of mean velocities. CVI needs only a few averaging calculations to reach a high degree of accuracy and consequently a high frame rate is maintained without reducing the number of color lines. This provides higher spatial and hemodynamic resolutions and more clinical informations.
As explained before, it is of importance that indications of the actual movements of the walls be available together with the blood flow velocity in order to diagnose stenoses or other diseases. However neither the CVI method nor the Color Doppler method provides indications of the radial velocity of the arterial walls or of the amplitude of the actual movements of the walls or of the dilation of the artery in function of the phases of the cardiac cycle.
A technical problem resides in the fact that processing data to provide arterial wall radial velocity together with processing data to provide CVI for example are actually incompatible because the respective velocities of arterial walls and blood flow are in the ratio of one to about fifty.